Not understanding how the hole plug works..
Drill a hole through the plastic syringe and use the 3 ml rubber plunger part to plug the hole. That way you don't have to use a syringe to fill. When it's time to fill, you just take the plug out, fill, replace the plug.
(I bought a Chopper Cyclone tank, and it uses the same type of plug. It works very well. My little earring stoppers work well, but the hole is a lot smaller than the one that uses the 3ml stopper. I still have to fill with a syringe, but I don't have to wedge the syringe in between the syringe and the plunger at least.)